EFFECT OF SILICON ON PRODUCING DUCTILE IRON WITH COMPLEX STRUCTURE OF BAINITE AND MARTENSITE

This paper deals with an important role of silicon in producing ductile iron with quenched complex structure of bainite and martensite. The samples are cast in permanent mold and quenched in solution of sodium silicate. The result of thc experiments shows that the austenizing temperature should rise with increasing silicon content, otherwise much undissolved ferrite is present in the matrix after quenching. However the undissolvec ferrite can be decreased greatly or even eliminated by adding appropriate amount of ooron. On this condition, the amount of bainite gets increasing and the amount of residual austenite decreasing with the silicon cortent increasing. An approach has also been made to the mechanism of the effect of silicon on the transformation of bainite in ductile iron. The T.T.T. curves measured show that the increase of sllicon content causes the curve to shift to the left. This is quite different from the fact in steel.

Effect of Austenitizing Treatment on Structure and Hardness of Bainite DuctileCast Iron

By continuous quenching process, the effect of austenitizing temperature and time on the structure and hardness of bainiteductile cast iron was studied. It was found that (l) low austenitizing temperature would result in scrap ferrite existing in matrix, whichreduces the macro-hardness of bainite ductile cast iron; (2) high austenitizing temperature would make carbide decomposed, which alsoinduces the macro-hardness of bainite cast iron, and (3) austenitizing time has little effect on the structure of bainite ductile cast iron, butas it increases, the macro-hardness ofbainite ductile cast iron and micro-hardness of bainite increases. To the ductile cast iron, as a result,the suitable austenitizing temperature and time are recommended as 880 and 120 min respectively.

Nucleation of Bainite in Bainite Ductile Cast Iron

The bainite ductile cast iron with given composition was quenched to get bainite structure.The nucleating position of bainite and the distribution of alloying elements in the matrix were measured.The results show that the bainite nucleates at the interface between graphite and austenite during quenching.Based on the experimental results and thermodynamics,the nucleating mechanism of bainite in ductile iron was analyzed.

A study on controlled cooling process for making bainitic ductile iron

In the present research, TTT curve of bainitic ductile iron under the condition of controlled cooling was generated. The cooling rate of grinding ball and its temperature distribution were also measured at the same time. It can be concluded that the bainitic zone of TTT curve is separated from the pearlitic zone. As compared to the water-quenching condition, more even cooling rate and temperature distribution can be achieved in the controlled cooling process. The controlled cooling can keep away from pearlitic zone in the high temperature cooling stage and produce similar results to the process of traditional isothermal cooling with a low cooling rate in the low temperature cooling stage.

Effects of Alloying Elements on the Microstructures and Mechanical Properties of Heavy Section Ductile Cast Iron

The effects of alloying elements on the as-cast microstructures and mechanical properties of heavy section ductile cast iron were investigated to develop press die material having high strength and high ductility. Measurements of ultimate tensile strength, 0.2% proof strength, elongation and unnotched Charpy impact energy are presented as a function of alloy amounts within 0.25 to 0.75 wt pct range. Hardness is measured on the broken tensile specimens. The small additions of Mo, Cu, Ni and Cr changed the as-cast mechanical properties owing to the different as-cast matrix microstructures. The ferrite matrix of Mo and Ni alloyed cast iron exhibits low strength and hardness as well as high elongation and impact energy. The increase in Mo and Ni contents developed some fractions of pearlite structures near the austenite eutectic cell boundaries, which caused the elongation and impact energy to drop in a small range. Adding Cu and Cr elements rapidly changed the ferrite matrix into pearlite matrix, so strength and hardness were significantly increased. As more Mo and Cr were added, the size and fraction of primary carbides in the eutectic cell boundaries increased through the segregation of these elements into the intercellular boundaries.

Low temperature impact strength of heavy section ductile iron castings:effects of microstructure and chemical composition

A foundry research project has been recently initiated at RTIT in order to better understand the fabrication of as-cast heavy section DI parts meeting high impact energy requirements at low temperatures.The experimental castings have the following dimensions 180 mm x 180 mm x 190 mm.The achieved as-cast Charpy impact strengths were as follows:17 J (RT),16 J (-20℃) and 11 J (-40℃).The foundry process,the chemical composition and the microstructure of this experimental casting are compared to the ones of various examples in order to show the detrimental effects of residual elements,microshrinkage and microcarbide on the impact properties.Finally,quality index empirical models (based on casting chemical compositions) are used to analyse the impact tests results.This paper illustrates that an adequate nodule count can contribute to reducing the detrimental effects of the residual elements and microsegregation.

Microstructure and Mechanical Properties of a Wear-Resistant As-Cast Alloyed Bainite Ductile Iron

An as-cast bainite ductile iron with excellent mechanical properties and wear resistance was fabricated by alloying and centrifugal casting method, and the alloyed chemical composition was optimized by using the thermodynamic software Thermo-Calc. By using optical microscopy, transmission electron microscopy, scanning electron microscopy, and X-ray diffraction, the microstructure of the as-fabricated bainite ductile cast iron was characterized pertinent to the elements distribution in matrix and features of ferrite and retained austenite. The results of mechanical properties test show that the hardness and compressive strength of this alloyed ductile iron are 52 HRC and 2,200 MPa, respectively. The as- cast bainite ductile iron possesses highly abrasive wear resistance and the reason can be ascribed to the solid solution of the elements Si, Ni, Cu, and Mn in the austenite and the formation of carbides of elements Cr and Mo. The strength of bainite ductile iron is increased by the acicular bainitic ferrite in the matrix.

As-Cast Acicular Ductile Aluminum Cast Iron

The effects of nickel （2.2 %） and molybdenum （0.6 % ） additions on the kinetics, microstrueture, and mechanical properties of ductile aluminum cast iron were studied under the as-cast and tempered conditions. Test bars machined from cast to size samples were used for mechanical and metallurgical studies. The results showed that adding nickel and molybdenum to the base iron produced an upper bainitic structure, resulting in an increase in strength and hardness. The same trend was shown when the test bars were tempered at 300 ℃in the range of 300 ℃ to 400 ℃. The elongation increased with increasing the temperature from 300 ℃ to 400 ℃. The carbon content of the retained austenite also increased with increasing the temperature. The results also showed that the kinetics, microstructure, and mechanical properties of this iron were similar to those of Ni-Mo alloyed silicon ductile iron.

铸造新材料——贝氏体球铁

贝氏体球铁与普通球墨铸铁以及铸钢锻钢相比,在抗拉强度、抗弯曲疲劳强度上具有明显的优势,应用前景良好,受到各国的重视和研究应用,国内的贝氏体球铁应用情况也呈良好的发展势头。本文从贝氏体球铁的生产制成、机械性能以及应用等方面,对贝氏体球铁做一个简单的介绍,希望对铸造行业的生产和发展有所启迪。

贝氏体球铁中位错密度的测定及其影响因素

采用Ｘ射线衍射法及新的线形分析理论，准确地测定了不同工艺条件下贝氏体球铁的位错密度．分析了化学成分、等温温度、回火处理及形变处理等与位错密度的关系．实验结果证明，位错密度的增加与贝氏体的晶粒细化伴生．

贝氏体球铁的研究与应用现状

贝氏体球墨铸铁由于具有优异的强韧性能和耐磨性能已得到广泛的应用。综合介绍了贝氏体球铁的研究、应用和发展概况。简要介绍了贝氏体球铁的各种生产方法及适用范围。

钒对贝氏体球墨铸铁组织和硬度的影响

研究钒对贝氏体球墨铸铁组织和性能的影响。研究结果表明 ,随着钒加入量的增加 ,贝氏体组织数量增加 ,贝氏体针尺寸明显减小 ,贝氏体球铁的宏观硬度提高 ,但钒对贝氏体球铁中奥氏体和贝氏体组织的显微硬度影响不大。测定了钒在贝氏体球铁中的存在形态 。

球墨铸铁的塑性变形及奥贝组织研究

利用Gleeble-1500热力学模拟机、光学和透射电子显微镜对球墨铸铁变形前后以及等温淬火处理后的组织进行了分析。结果表明，球铁热塑性变形最佳温度在750—800℃区间，此时流变应力接近950℃时的流变应力；变形后球铁为三层组织结构“球状石墨＋蠕虫状石墨＋流线型石墨”；在透射电镜下清楚地观察到不同形状的下贝氏体和上贝氏体形貌以及典型的隐“M”型马氏体形貌。

低合金贝氏体球墨铸铁磨球热处理工艺参数的确定

研究了室温油分级等温淬火热处理工艺参数对低合金贝氏体球墨铸铁磨球组织和性能的影响。结果表明：试验条件下最佳热处理工艺参数为奥氏体化温度930～950℃,出油温度180～220℃,出油入炉时间60 s,等温温度220～240℃,等温时间1～1．5 h；此时材料基体组织为下贝氏体+少量的马氏体、残余奥氏体和碳化物,其冲击韧度大于14J／cm^2,硬度为52～56 HRC。

Cu、Mo对水平连铸球铁型材组织与力学性能的影响

在相同水平连铸条件下,采用合金化方法研究了不同Cu和Mo含量对60mm球铁型材的基体组织与力学性能的影响。结果表明,加入一定量的Cu和Mo元素,可以获得珠光体型球铁组织或奥氏体-贝氏体型球铁,在Mo含量不变时,获得铸态奥氏体-贝氏体基体组织主要是由Cu含量决定。加入Cu和Mo元素后,球铁型材的抗拉强度、屈服强度和硬度均得到较大提高,但冲击韧度明显下降。

准铸态贝氏体低碳球铁的组织分析

采用扫描电镜和X射线衍射仪对准铸态贝氏体低碳球铁的组织结构、断口形貌和微区成分进行了分析和研究.结果表明,经准铸态贝氏体工艺处理后,低碳球铁的基体组织以针状无碳贝氏体为主,并有25%～28%的奥氏体;准铸态贝氏体低碳球铁韧塑性的提高,与其组织中形成的大量奥氏体和镶嵌在当中的针状无碳贝氏体密切相关.

具有性能梯度特性的贝-马-奥球墨铸铁磨球的制备工艺

采用10 kg真空中频感应炉熔炼铁液,经炉前一次球化及两次孕育处理,真空浇注得到球墨铸铁球。使用电热炉加热磨球到不同的奥氏体化温度,并在不同温度的硝酸盐和亚硝酸盐混合介质中进行连续冷却淬火,后经低温回火得到球墨铸铁磨球。其中,性能最佳的为贝-马-奥球墨铸铁磨球,其显微组织为:球状石墨(体积分数12%左右)+下贝氏体+高碳针状马氏体+残留奥氏体(质量分数16.6%~18.1%)。石墨球化率达到92%、球化级别为2级,球墨尺寸级别为9.2级;磨球表面硬度为55~58 HRC,表面与心部硬度差【5 HRC;磨球表层的常温冲击韧性为20.5 J/cm2,心部冲击韧性为25.3 J/cm2,表现出表层高硬度,耐磨损;心部高韧性,抗破碎的性能梯度特性。进一步研究表明:下贝氏体和残留奥氏体组织对提高磨球的性能有着显著的作用,而上贝氏体组织则不利于磨球的综合性能。

抗磨贝氏体球墨铸铁的研究与应用

在Mn-Si球铁中加入Cu、Cr、B、Nb等合金元素,采用分级等温淬火工艺,用金属型铸造代替传统砂型铸造生产抗磨贝氏体球铁。结果表明：与传统方法生产的抗磨贝氏体球铁比较,新工艺使磨球的硬度提高3～5HRC,冲击韧度提高10%～20%,耐磨性提高20%～30%,磨球破碎率≤1%。当碳化物含量大于12%时,磨球的破碎倾向显著提高。

钒对贝氏体球墨铸铁组织与硬度的影响

研究了钒在连续冷却淬火贝氏体球墨铸铁中的存在形式和作用，以及对球墨铸铁贝氏体组织和性能的影响机制．结果表明：钒在贝氏体球墨铸铁中以固溶和弥散碳化物２种形式存在，它能有效地增加贝氏体形核率，促进贝氏体转变，细化贝氏体组织，强化基体，提高硬度．因此，钒可替代钼、镍、硼等元素生产贝氏体球铁．

奥贝型球铁组织形貌特征

论述了钢、球铁等淬贝氏体和连续冷却所得组织的特征，并在电镜下观察，证明球铁中上贝氏体针内有与针体成长方向大致平行的断续的短杆状碳化物，在下贝氏体针内有不均匀分布的点状碳化物。在岩棉中连续冷却得到奥贝型组织，４６ＨＲＣ、 α_Ｋ＞２６Ｊ／ｃｍ~２，宜作抗磨材料。用水玻璃作介质，获得Ａ＋Ｂ_下＋Ｍ组织，使材料具有更高的硬度。